An unsaturated compound such as perfluoro(alkyl alkenyl ether) is useful as a starting material monomer for a fluororesin. A fluororesin has excellent properties in respect of heat resistance and chemical resistance, and it is therefore widely used. Heretofore, perfluoro(alkyl alkenyl ether) is industrially prepared by dimerization reaction of perfluoroepoxides or by reacting perfluoroalkanoic acid fluoride with perfluoroepoxides in the presence of an alkali metal fluoride to form perfluoro(2-alkoxyalkanoic acid)fluorides, followed by heating reaction in the presence of glass beads or an alkali metal salt such as soda ash (Methods of Organic Chemistry, 4, Vol. 10b, Part 1, p. 703, etc.)
Also, a reaction of obtaining an acid fluoride compound by heating a perfluorinated alkyl ester compound having a carbon number of at least 16 is known as a reaction of pyrolyzing an ester bond (J. Am. Chem. Soc., 120, 7117 (1998)).